1. Field of the Invention
The present invention relates to an apparatus mounted on a vehicle for starting an engine mounted on the vehicle.
2. Description of Related Art
In recent years, there has been developed and put to practical use a so-called idle stop system which automatically stops an engine of a vehicle when the engine is not needed to run, for example, while the vehicle stops at a red light, and automatically restarts the engine when the engine becomes needed to run, in view of reduction of fuel consumption and discharged exhaust gas.
The vehicle provided with such an idle stop system is strongly required that the starter thereof is sufficiently quiet and durable, because its engine stops and restarts at frequent intervals. To address this requirement, there is proposed a technique in which a pinion-engagement controlling solenoid which operates to make engagement between a pinion of the starter and a ring gear of the engine is configured to be individually controllable by an ECU (electronic control unit), and the pinion is engaged with the ring gear while the engine is stopped in order to reduce shock at the time when the engagement is made. For more details, refer to Japanese Patent Application Laid-open No. 2001-317439, or Japanese Patent Application Laid-open No. 11-30139.
In this technique, a solenoid relay for operating the pinion-engagement controlling solenoid and a motor relay for operating a starter motor are individually controlled by the ECU. During idle stop, the ECU energizes the solenoid relay so that the pinion-engagement controlling solenoid operates to engage the pinion with the ring gear, and thereafter, when engine starting conditions are satisfied, energizes the motor relay to operate the starter motor to restart the engine. According to this technique, since timings to drive the relays of the starter can be controlled individually and minutely by the ECU, quietness and durability of the starter can be improved.
Meanwhile, when the starter motor is started, since a large current flows through the starter motor because the starter motor has to generate large torque to rotate the crankshaft of the engine, the voltage of a vehicle battery drops.
Usually, when the engine is restarted after idle stop, lubricating oil of the engine is warm enough because the engine was running immediately before the idle stop. Accordingly, since the power which the starter motor needs to restart the engine is comparatively small, and accordingly, the battery voltage does not drop so much. Ordinarily, the battery voltage of 12 V does not drop below 6 V.
However, when the engine is started not after idle stop, but after the vehicle has been parked (referred to as “normal engine start” hereinafter), since the lubricating oil of the engine is cold, the starter motor is required to generate torque to rotate the engine which is larger when the lubricating oil is warm. Accordingly, in this case, the battery voltage may drop as low as 3.5 V, for example.
Generally, the ECU includes a power supply circuit which generates a control-use voltage at a constant level (5V, for example) from the battery voltage. A microcomputer included in the ECU to perform various controls of the idle stop system including controls of the pinion-engagement controlling solenoid and the starter motor operates on this control-use voltage.
The power supply circuit can stably generate the control-use voltage of 5 V unless the battery voltage of 12 V drops below a lower limit of its operation-guaranteed voltage range (6 V, for example). However, if the battery voltage drops below the lower limit, the control-use voltage generated by the power supply circuit becomes lower than 5 V. For example, if the battery voltage drops to 4.5 V and below, the control-use voltage generated by the power supply circuit is lowered to 3.9 V and below.
Accordingly, the microcomputer is configured to be reset when the battery voltage becomes low voltage (4.5 V for example), and consequently the control-use voltage falls below a predetermined voltage (3.9 V, for example), in order to prevent malfunction from occurring due to the fall of the control-use voltage.
Hence, there has been a risk that if the battery voltage drops excessively during normal engine start operation, the control-use voltage is lowered excessively, causing the microcomputer to be reset, and accordingly causing each relay of the starter to be uncontrollable. As explained above, the techniques disclosed in the above patent documents in which the microcomputer included in the ECU individually controls the solenoid relay and the motor relay have the risk that the microcomputer is reset causing it to be unable to control these relays and accordingly causing it unable to start the engine, if the battery voltage drops excessively during the normal engine start operation.
Incidentally, as a measure to prevent the microcomputer from being reset when the battery voltage drops excessively causing the control-use voltage to be lowered excessively, it is known to provide a voltage step-up circuit for stepping the battery voltage in the ECU. However, providing such a voltage step-up circuit increases manufacturing costs.